An optical transmitter modulates an input signal having a predetermined waveform with the use of an optical modulator, and outputs the modulated signal as optical power having a predetermined waveform. An optical output waveform depends largely on a waveform of an input signal, and therefore, it is difficult for the optical transmitter to be sufficiently tolerant to variation in the amplitude of an input signal. Consequently, conventionally, the optical transmitter generates a signal independent of amplitude variation with the use of an interface circuit or the like, and then causes the optical modulator to compensate a waveform which has been degraded through a modulator drive circuit. Thus the optical transmitter can obtain a uniform optical output waveform regardless of the amplitude of an input signal. Such a configuration as described above is often adopted especially for an optical transmitter mounted with an electro absorption (EA) modulator aimed at long-distance transmission.
Patent document 1: Japanese Laid-open Patent Publication No. 2003-163639
Patent document 2: International Publication Pamphlet No. WO 2006/048944
Patent document 3: Japanese Laid-open Patent Publication No. 10-27931
Patent document 4: Japanese Laid-open Patent Publication No. 2000-221457
However, when the above-described interface circuit is mounted on the optical transmitter, packaging efficiency is reduced, and it becomes difficult to achieve downsizing by high-density packaging and low power consumption. Therefore, an optical transmitter without an interface circuit is desired; however, such an optical transmitter does not generate a signal independent of variation in the amplitude, and variation in the amplitude directly appears as variation in an optical output waveform. As a result, the tolerance to amplitude variation is significantly reduced. Especially, when the amplitude of an input signal is small, a shift amount of a cross-point in a waveform of a signal input to an optical modulator is not consistent with extinction characteristics of the optical modulator, and the cross-point sometimes does not fall within an allowable range (on or around 50%) in the optical output waveform. This is a factor that inhibits a stable coding error characteristic before and after long-distance transmission.